Air Source Heat Pumps

Heat pumps extract thermal energy from outside sources such as the air or the ground and use it to heat our buildings – and with some designs of air source heat pumps able to absorb heat from external temperatures as low as minus 15 degrees, interest in these systems is growing.

The principle is one that we’re all familiar with – heat pumps work the same way a fridge does – and being energy efficient, they produce three or four units of heat for every unit of electricity used. Heat pumps can help reduce a household’s fossil fuel use – and its carbon footprint.

How Does It Work?

Think of heat pumps as fridges in reverse. Instead of extracting heat from interior, to make the inside of the fridge cool, they bring heat in from the outside, to make it warm indoors. There are three main parts to an air source heat pump. Firstly there is an evaporator coil – mounted outside – which does the actual heat absorbing part.

A compressor unit then drives refrigerant through the heat pump and compresses it to the right level to suit the heat distribution system; in your kitchen refrigerator, this is the part that makes the noise. Finally there is a heat exchanger which transfers the heat from the refrigerant to the air or water for use, depending on the type of system installed.

Choosing Heat Pumps

There are two sorts of air source heat pumps – air-to-air and air-to-water. As their name suggests, air-to-air heat pumps are used to produce warm air, which is then circulated by fans to provide space heating within the building.

Air-to-water systems can provide heat for under-floor heating systems or radiators, or alternatively they are ideal for pre-heating water in a storage tank, since they work at a lower temperature than a conventional boiler.

These heat pumps are suitable for a wide range of different kinds of applications and can be used in individual homes or larger community buildings – and the more energy efficient the building, the greater the rewards.

According to the government’s Low Carbon Buildings Programme, a typical 5kW domestic installation to heat a well insulated detached house would cost somewhere between £6,000 to £8,000, plus VAT. Obviously the pay-back is best when they are used to replace relatively expensive fuel sources such as coal, electricity, oil or LPG – making them a particularly good choice for rural or remote locations where no mains gas supply is available.

Although air source heat pumps have been used in other parts of the world for some time, they are a relatively new entrant onto the UK energy scene; ground source heat pumps – a close relative – are much more familiar.

However, air source heat pumps are becoming increasingly popular, largely because without the need for extensive excavation, they are more easily installed than their ground source equivalents and require far less space. Never-the-less, you do need some space on an external wall to accommodate the evaporator coil the system requires.

There’s a tremendous appeal in the idea of getting something for nothing and being able to warm your house with heat from the great outdoors sounds just too good to be true. Of course it isn’t really “free” – you’ll have to pay for the electricity to drive the pump, not to mention the installation in the first place. However, there are even things you can do to help with these.

Combining your heat pump with a micro-generation unit or a green electricity tariff could help, while air source heat pumps can qualify for a Low Carbon Buildings Programme grant towards their cost. It may not be exactly free energy, but it’s probably about as close as you are ever going to get – making them an option well worth considering.